The present invention relates to semiconductor processing. More specifically, the invention relates to methods and apparatus for forming films at temperatures greater than about 400.degree. C. in a corrosive environment with a plasma. In some specific embodiments, the invention is useful for forming titanium-containing films such as titanium, titanium nitride, and titanium disilicide at temperatures of up to about 625.degree. C. or greater. Such films may be used as patterned conductive layers, plugs between conductive layers, diffusion barrier layers, adhesion layers, and as a precursor layer to silicide formation. In addition, other embodiments of the present invention may be used, for example, to deposit other types of metal films, to alloy substrate materials, and to anneal substrate materials.
One of the primary steps in fabricating modem semiconductor devices is forming various layers, including dielectric layers and metal layers, on a semiconductor substrate. As is well known, these layers can be deposited by chemical vapor deposition (CVD) or physical vapor deposition (PVD). In a conventional thermal CVD process, reactive gases are supplied to the substrate surface where heat-induced chemical reactions (homogeneous or heterogeneous) take place to produce a desired film. In a conventional plasma CVD process, a controlled plasma is formed to decompose and/or energize reactive species to produce the desired film. In general, reaction rates in thermal and plasma processes may be controlled by controlling one or more of the following: temperature, pressure, plasma density, reactant gas flow rate, power frequency, power levels, chamber physical geometry, and others. In an exemplary PVD system, a target (a plate of the material that is to be deposited) is connected to a negative voltage supply (direct current (DC) or radio frequency (RF)) while a substrate holder facing the target is either grounded, floating, biased, heated, cooled, or some combination thereof. A gas, such as argon, is introduced into the PVD system, typically maintained at a pressure between a few millitorr (mtorr) and about 100 mtorr, to provide a medium in which a glow discharge can be initiated and maintained. When the glow discharge is started, positive ions strike the target, and target atoms are removed by momentum transfer. These target atoms subsequently condense into a thin film on the substrate, which is on the substrate holder.
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two-year/half-size rule (often called "Moore's Law") which means that the number of devices which will fit on a chip doubles every two years. Today's wafer fabrication plants are routinely producing 0.5 .mu.m and even 0.35 .mu.m feature size devices, and tomorrow's plants soon will be producing devices having even smaller feature sizes. As device feature sizes become smaller and integration density increases, issues not previously considered crucial by the industry are becoming of greater concern. For example, devices with increasingly high integration density have features with high aspect ratios (for example, about 6:1 or greater for 0.35 .mu.m feature size devices). (Aspect ratio is defined as the height-to-spacing ratio of two adjacent steps.) High aspect ratio features, such as gaps, need to be adequately filled with a deposited layer in many applications.
Increasingly stringent requirements for fabricating these high integration devices are needed and conventional substrate processing systems are becoming inadequate to meet these requirements. Additionally, as device designs evolve, more advanced capabilities are required in substrate processing systems used to deposit films made of materials needed to implement these devices. For example, the use of titanium is increasingly being incorporated into integrated circuit fabrication processes. Titanium has many desirable characteristics for use in a semiconductor device. Titanium can act as a diffusion barrier between, for example, a gold bonding pad and a semiconductor, to prevent migration of one atomic species into the next. Also, titanium can be used to improve the adhesion between two layers, such as between silicon and aluminum. Further, use of titanium, which forms titanium disilicide (silicide) when alloyed with silicon, can enable, for example, formation of ohmic contacts. A common type of deposition system used for depositing such a titanium film is a titanium sputtering deposition system which is often inadequate for forming devices with higher processing and manufacturing requirements. Specifically, sputtering may result in damaged devices which suffer from performance problems. Also, titanium sputtering systems may be unable to deposit uniform conformal layers in high aspect ratio gaps because of shadowing effects that occur with sputtering.
In contrast to sputtering systems, a plasma-enhanced chemical vapor deposition (PECVD) system may be more suitable for forming a titanium film on substrates with high aspect ratio gaps. As is well known, a plasma, which is a mixture of ions and gas molecules, may be formed by applying energy, such as radio frequency (RF) energy, to a process gas in the deposition chamber under the appropriate conditions, for example, chamber pressure, temperature, RF power, and others. The plasma reaches a threshold density to form a self-sustaining condition, known as forming a glow discharge (often referred to as "striking" or "igniting" the plasma). This RF energy raises the energy state of molecules in the process gas and forms ionic species from the molecules. Both the energized molecules and ionic species are typically more reactive than the process gas, and hence more likely to form the desired film. Advantageously, the plasma also enhances the mobility of reactive species across the surface of the substrate as the titanium film forms, and results in films exhibiting good gap filling capability.
However, conventional PECVD systems which use aluminum heaters may experience some limitations when used for certain processes, such as forming a titanium film from a vapor of, for example, titanium tetrachloride (TiCl.sub.4). Aluminum corrosion, temperature limitations, unwanted deposition, and manufacturing efficiency are some of the problems with such conventional PECVD systems that may be used to deposit a film such as titanium. In the exemplary process, titanium tetrachloride, which is a liquid at room temperature, and a carrier gas, such as helium, bubbled through this liquid generates vapor that can be carried to a deposition chamber. Such a titanium PECVD process may require a substrate temperature of about 600.degree. C. to achieve a deposition rate of about 100 .ANG./min, which may be insufficient to achieve good wafer throughput. However, when the titanium tetrachloride disassociates to form the titanium film, chlorine is released into the chamber. In particular, the plasma, which enhances the titanium film deposition, forms chlorine atoms and ions that undesirably tend to corrode aluminum heaters and other parts of the chamber, such as the faceplate under these conditions. The aluminum corrosion may also lead to processing degradation issues relating to metal contamination in the devices. Additionally, use of a PECVD system having an aluminum heater is limited to operation at temperatures less than about 480.degree. C., which may therefore limit the film deposition rates that can be achieved. Aluminum is an inappropriate material for heaters operating at high temperature, because at temperatures greater than about 480.degree. C. aluminum heaters experience softening, possibly resulting in warpage of and/or damage to the heater. Additional problems arise when aluminum heaters are used above about 480.degree. C. in the presence of a plasma. In such an environment, the aluminum may backsputter, contaminating the substrate and chamber components. Furthermore, aluminum heaters (and other parts of the chamber such as the faceplate), which tend to be incompatible even at lower temperatures with some of the chemical species associated with some deposition processes (such as the chlorine compounds produced in a titanium deposition process), experience greatly increased attack at higher temperatures. Chemical species, such as chlorine, used in dry clean processes also attack the aluminum heaters. At temperatures higher than about 480.degree. C., these chemical species may more aggressively attack and corrode aluminum heaters than at lower temperatures, thereby reducing the operational lifetime of the heater and undesirably requiring more frequent heater replacement. Heater replacement is expensive not only because of the cost of the heater, but also because the productive use of the deposition chamber is lost for the time the heater is being replaced. During such dry clean processes, a dummy wafer is often loaded onto the aluminum heater to try to minimize the attack on the heater. However, loading and unloading of the dummy wafer consumes time and decreases wafer throughput. Also, some dummy wafers, which get attacked by the dry clean chemistries, are expensive and may need periodic replacement, which adds to the overall maintenance costs.
In addition to aluminum corrosion, heater softening, and temperature limitations, other concerns with metal depositions in a PECVD processing system include unwanted metal deposition and related manufacturing efficiency problems. While the greatest film deposition generally occurs in places where the temperature is the highest, some deposition will occur at lower temperatures, even in the absence of a plasma. Unwanted metal depositions can cause multiple problems, such as uneven deposition, arcing, degraded operation of chamber components, and/or device defects. Besides occurring on chamber wall and bottom surfaces, unwanted metal deposition may occur on non-conductive components, such as ceramic spacers and liners within the deposition chamber or chamber exhaust path, which then become conductive. This undesired conductive metal deposition can disrupt the shape of the glow discharge, resulting in uneven deposition across the substrate. It can also cause arcing, which may damage the substrate and parts of the chamber such as the faceplate. Further, titanium may build up on parts of the heater, in gas or vacuum apertures to undesirably restrict the flow therethrough, or on mechanical parts having close tolerances to interfere with their operation. Unwanted deposits that have bonded poorly to the underlying chamber component or that have built up on the heater may result in flakes and other particles that fall onto the substrate and cause defects on the substrate, thus reducing substrate yield. For these and other reasons, the chamber must periodically be cleaned with dry clean processes, which do not require opening of the chamber, and a preventive maintenance clean, which requires at least partially disassembling the chamber and wiping it down. The chamber may be cleaned in various ways. A "dry" cleaning process may use reactive gas or plasma species to etch unwanted deposits from the chamber components, or may physically bombard particles with plasma species to knock them loose, to be removed by the exhaust system. A "wet" cleaning process may be done in addition or as an alternative to a dry clean. A wet clean typically involves at least partial disassembly of the chamber, which is then wiped down with solvents.
Subsequently, the chamber must be reassembled and may be "seasoned", i.e., a number of deposition cycles must be performed until consistent layers are obtained. Both procedures take the deposition system out of productive operation, which is inefficient and uneconomic, but wet cleans usually decrease throughput more than dry cleans. Therefore, it is desirable to have an efficient dry clean process to minimize the frequency of wet cleans so that more wafers may be produced between cleans. It is also desirable to minimize the areas in the chamber upon which unwanted depositions may form. In some deposition processes, particularly metal depositions such as tungsten or titanium, the time required for cleaning the chamber becomes a major factor affecting the deposition system's wafer output.
Although ceramic heaters have been proposed as an alternative to using aluminum heaters for deposition systems operating at or above about 400.degree. C., fabricating ceramic heaters and using them in deposition processes introduce several challenges. Such ceramic heaters advantageously may be used in the presence of plasma and corrosive plasma species, such as chlorine-containing species found in a titanium PECVD process and associated cleaning processes. Ceramic heaters typically have an electric heating element within a ceramic heater body, made of materials such as alumina (Al.sub.2 O.sub.3) or aluminum nitride (AlN), which protects the heating element from the corrosive environment of the deposition chamber while transmitting heat from the heating element to the substrate. Typically harder and more brittle than metals, ceramic materials may be difficult to machine, thereby requiring a simple mechanical design. Being somewhat brittle, ceramic may crack from thermal shock if repeatedly subjected to a sufficient thermal gradient. Cracking may also arise from the differential thermal expansion at the transition from the ceramic heater assembly to a material with a different thermal expansion coefficient. Even joining ceramic parts fabricated from the same material is a challenge because many assembly methods and devices used to assemble metal parts, such as welding, bolting, brazing and screwing, may be unreasonably difficult or unreliable when attempted with ceramic parts.
In light of the above, improved methods, systems and apparatus are needed for efficient plasma-enhanced deposition of films in a high temperature (at least about 400.degree. C.), corrosive environment. Optimally, these improved methods and apparatus will require less chamber cleaning and result in higher substrate output. In particular, these systems and methods should be designed to be compatible with processing requirements for forming devices with high aspect ratio features.